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Hard carbon baked onto the back of the intake valves, which disrupts airflow and fuel preparation into that cylinder. It is overwhelmingly a direct-injection problem, and the reason is structural rather than a defect: on a port-injected engine petrol sprays across the valve back and washes it continuously, while a direct-injection engine sprays into the cylinder and nothing ever cleans the valve at all.

The deposit is fed by what the valve does see — oil mist from the crankcase breather and recirculated exhaust — so it builds steadily from new and is normal by mid-life. It disturbs cylinders unequally, which is what makes it a misfire and knock cause rather than a general performance one: the cylinder with the worst deposit runs leaner and more turbulently than its neighbours, so a single-cylinder fault appears on an engine where nothing is broken.

It does not respond to fuel additives, whatever the bottle claims, because the fuel never touches the deposit. Removal is mechanical — walnut blasting or manual cleaning with the manifold off. Check it before condemning an injector or a coil on a direct-injection engine with a stubborn single-cylinder misfire, since the deposit is visible with a borescope and costs nothing to look at.